A nitride semi-conductor light emitting device which emits light in UV-visible region is expected to be widely applied, hygienically, medically, industrially, and has been studied to develop an illumination device, a precision machine or the like, for its compact size and its low electrical consumption. The light emitting device which emits light in specific region including blue-color region has already been developed for practical use.
For development of the nitride semi-conductor light emitting device (including that emitting blue light which is referred to as a blue-color light emitting diode), it is necessary to further improve its light emission efficiency and light output. At present, the semi-conductor light emitting device provides much smaller external quantum efficiency and light output in UV region than blue-color region. The very small external quantum efficiency and light output have been obstacles to practical use of the light emitting device in UV region, and are possibly attributed to very low light emission efficiency in light emission layer (referred to as internal quantum efficiency, hereafter) and high resistance in an n-type and p-type nitride semi-conductor layers.
Due to transition, point defect, and unintended impurity which are formed in large amount in the light emission layer, the nitride semi-conductor (nitride mixed crystal) has very low internal quantum efficiency. Especially, ternary mixed crystal including Al such as AlGaN has considerably low internal quantum efficiency, for difficulty in growth of the mixed crystal with high quality. Instead, a quaternary mixed crystal AlGaInN has been attracted as an mixed crystal which is less affected by the transition and the point defect than AlGaN, for improving the internal quantum efficiency and light intensity.
A UV LED has been proposed for being utilized as a component of the nitride semi-conductor light emitting device. The UV LED is designed to emit light in 280 to 360 nm at room temperature. The UV LED as the light emitting device has an AlaGabIncN(0.1≦a≦0.9, 0.02≦c≦0.2, a+b+c=1) layer grown at growth temperature of 830 to 950° C. A UV LED of JP unexamined patent publication 2005-340856 has an AlGaInN layer which emits light with higher intensity than that emitted from a conventional AlGaN layer. The growth rate of the AlGaInN layer is set at 0.12 μm/h by metal organic vapor phase epitaxy (MOVPE) method.
However, the internal quantum efficiency in the light emission layer is still low in this LED in JP unexamined patent publication 2005-340856, and required to be further improved.
The Al-containing nitride semi-conductor layer is generally grown at a high temperature of 1000° C. or more. For example, growth temperature is generally set at 1150° C. or more for growth of AlN layer, or 1000 to 1200° C. for growth of AlGaN layer.
In the UV LED including AlGaInN layer of JP Unexamined patent publication 2005-340856, the growth temperature needs to be set at 830 to 950° C. below 1000° C. When the growth rate is set at 0.12 μm/h under such a low temperature, Al atoms are not migrated sufficiently through a surface of single crystal substrate in its crystal growth, thereby often generating voids and point defects particularly in growth of AlGaInN layer having a high concentration of Al. The point-defect causes a sharp decrease in the internal quantum efficiency.